Rectifier electroforming process



July 28, 1959 G. EANNARINO 2,896,305

RECTIFIER ELECTROFORMING PROCESS Filed Dec. 16, 1954 INVEN TOR. $502 5 54/V/VAE/NO BY United States Patent 59 RECTIFIER ELECTROFORMING PROCESS George Eannarino, Bloomington, Ind., assiguor to Sarkes Tarzian, Inc., Bloomington, Ind., a corporation of Indiana Application December 16, 1954, Serial No. 475,705

7 Claims. (Cl. 29-253) This application is a continuation-in-part of my copending application Serial No. 394,756, filed November 27, 1953, now abandoned.

The present invention relates to rectifiers, more particularly, to rectifiers of the dry surface contact type, and the invention has for an object the provision of an improved rectifier of the above-described type which operates at cooler plate temperatures, has longer life and may be manufactured in a much shorter length of time than conventional rectifiers. While the present invention is of general utility in the dry surface contact rectifier field, it is particularly suited for and will be described in connection with the manufacture of selenium rectifiers, particularly selenium rectifiers of the type used in the radio and television industry.

In accordance with present day manufacturing techniques of selenium rectifiers of the above-described type it is customary to subject the selenium rectifier to a socalled electroforming operation after the rectifier has been mechanically completed. This electroforming operation is performed for the purpose of increasing the reverse or back resistance of the rectifier without greatly affecting the forward resistance thereof so that maximum asymmetry is achieved and the rectifying characteristic of the rectifier is enhanced. While the electroforming operation is very desirable from the standpoint of improving the ratio of reverse to forward resistance of the rectifier, it nevertheless requires from two to four hours to complete the electroforming operation by conventional techniques so that a large portion of the manufacturing time involved in producing the rectifier is expended in the electroforming operation.

Accordingly, it is a further object of the present invention to provide a new and improved method of electroforming selenium rectifiers whereby the electroforming operation may be completed in a relatively short period of time.

It is a further object of the present invention to provide a new and improved method of electroforming selenium rectifiers at constant or fixed maximum voltage wherein the rectifier is electroformed at a relatively high current density and at low temperatures to provide cooler plate operating temperature and a longer life characteristic therefor.

It is another object of this invention to provide an improved method of electroforming selenium rectifiers wherein the rectifier is electroformed at a relatively high current density of the order of magnitude of 1 to 3 amperes per square inch of rectifier surface, at a low bath temperature below 40 C., and at a fixed voltage across the circuit so that it is unnecessary to constantly regulate the voltage and hence the current density. This method allows for production-line operation under fixed conditions without constant supervision and adjustment.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to Patented July 28, 1959 the following specification taken in connection with the accompanying drawings, in which:

Fig. 1 is a perspective view of a rectifier cell which may be manufactured in accordance with the principles of the present invention;

Fig. 2 is a side elevational view of a half wave stack of rectifier cells which may be manufactured in accord ance with the principles of the present invention;

Fig. 3 is an electrical circuit diagram illustrating the manner in which single rectifier cells may be energized while being electroformed in accordance with the present invention;

Fig. 4 is an electrical circuit diagram of an alternative arrangement which may be employed for electroforming single rectifier cells; and

Fig. 5 is an electrical circuit diagram illustrating the manner in which a half wave rectifier stack may be energized While being electroformed in accordance with the principles of the present invention.

Referring now to the drawing, there is illustrated in Fig. 1 atypicaldry surface contact rectifier cell 10 which may be manufactured in accordance with the principles of the present invention. .While the present invention relates to the electroforming operationwhich may be performed on a single rectifier cell or a stack of assembled cells after the cells have been mechanically completed, a brief description of the manner in which p a typical selenium rectifier cell is manufactured up to the electroforming operation is included to facilitate an understanding of the present invention. However, it will be understood that the el'ectroforming operation of the present invention may be equally well performed on dry surface contact rectifiers of other constructions. Thus, the cell 10 comprises an aluminum base plate 11 which is roughened by chemical etching, or the like, and a thin film 12 of nickel is deposited on one surface of the aluminum base plate 11 by electroplating or other suitable deposition method. A layer of selenium 13 is deposited on the surface of the nickel plated layer 11 under heat and pressure, and after suitable heat treatment the surface of the selenium layer 13 is covered with a thin film of lacquer 14, commonly called an artificial barrier layer, which has the effect of increasing the maximum operating voltage of the cell without producing an excessive increase in the forward resistance of the cell. A counter electrode 15 is. then formed by metal spraying a low melting point alloy on top of the barrier layer surface 14. After the rectifier cell 10 has been formed it may be assembled in a half wave rectifier stack as shown in Fig. 2. Thus, the rectifier cells 20 are positioned on a tubular center post 21 of suitable insulating material and are spaced apart and electrically connected together by means of the metal corrugated washers 22, the plates 20 being held in place by means of the internal toothed retaining washers 23 and 24 at the ends of the rectifier stack, and the terminals 25 and 26 also being held in place by the retaining washers 23 and 24.

In accordance with the present invention either a rectifier cell 10 such as is shown in Fig. 1 or a half wave rectifier stack such as is shown in Fig. 2 may be electroformed in a relatively short period of time so that maximum asymmetry is achieved with a minimum expenditure of manufacturing time. Thus, considering first the single rectifier cell operation, after these cells have been mechanically completed they are soaked in a bath of oil.

Various oils may be used for this purpose. Examples of oils found to be suitable include such oils as transformer oil and spindle oil. The oil in this bath is warm, being maintained preferably at a temperature of the order of about C. Generally when the oil is at approximately this temperature a soaking time of the order of five minutes gives results which are entirely satisfactory. Either longer or shorter soaking times may be used, however, bearing in mind that decreasing the time substantially tends to efiect the results of the soaking adversely whereas substantial increases in time of soaking though not generally harmful do not usually produce any great improvement.

As in processes heretofore known the oil used in this soaking treatment preferably contains an alkaline material. Better results are obtained if the oil containing the alkaline material has a pH of from about 7.2 to about 8.0.

Among the various alkaline materials which may be used in the oil, it has been found that amines and similar compounds of nitrogen and particularly aliphatic amines and hydroxy amines give the best results. By way of example, trihydroxytriethylamine, ethanolamine, and dihydroxydiethylami'ne all give good results.

By way of illustration a suitable bath is prepared by mixing diethylenetriamine with transformer oil in the proportion of /2 pint of amine to a gallon of oil. Since the characteristics of the oils vary somewhat, the pH of the mixture so obtained is checked and if it is not at the desired level, for example a pH of 8.0, it is adjusted by adding more oil or more amine depending upon whether it is too high or too low. The alkaline materials such as amines are consumed to some extent during the soaking and in accordauce'with the practice heretofore followed the pH of the bath should be checked periodically (for example, at /2 hour intervals) and if it is not at the desired level additional quantities of the amine or other alkaline material should be added to replace what has been consumed and restore the desired pH level.

Following the above-described soaking treatment the rectifier cells are preferably transferred immediately to a cold oil bath usually of the same type as the warm oil bath in that the same kinds of oils may be used in this bath and in that the bath also contains an amine or other alkaline material. Preferably, however, the pH of this bath is maintained approximately at neutrality (pH 7.0). Periodic determination of pH in this bath is also made and amine or other alkaline material consumed is replaced to restore the bath to approximate neutrality. The temperature of this bath is preferably maintained below 40 C. Since heat is produced during the treatment of the cells in this bath, the desired bath temperature is maintained by cooling means. Any desired cooling means may be employed. For example, Dry Ice may be added to the bath or the bath may be refrigerated by cooling coils through which a refrigerant is circulated. I

When the rectifiers are placed in the cold oil bath they are connected in an electrical circuit in the manner shown in Fig. 3. Thus, referring to this figure, a plurality of rectifier cell pairs, such as the rectifier cells 30 and 31 are connected in parallel across the secondary of a suitable power transformer 32 to the primary of which there is applied an alternating current of constant voltage. The base plates of the rectifier cells 30 and 31 are connected together so that the rectifiers are connected back to back across the secondary of the transformer 32. With this arrangement one cell of each pair receives substantially the full applied voltage in the reverse direction on alternate half cycles of the applied voltage since the other rectifier of the pair is fully conducting during these periods. Thus, substantially the full applied voltage is impressed upon the rectifiers 30, 33 and 35 in the reverse direction when the conductor 37 is negative with respect to the conductor 38 since under these conditions the rectifiers 31, 34 and 36 are fully conducting so that a relatively small voltage drop is produced across the rectifiers 31, 34 and 36. Resistor .39, of the order of /2 ohm, is connected in series with the pairs of rectifiers to absorb the initial surge of ourrent which occurs when a constant voltage is impressed across the primary of the transformer, resulting in a constant voltage E across the secondary.

After the rectifier cell pairs are placed in the cold oil bath and are connected as shown in the electrical circuit of Fig. 3 the voltage E is applied across the rectifier cell pairs so that the rectifier cells are initially operated at a relatively high current density in the reverse direction, preferably in the order of three amperes per square inch. As the back resistance of the rectifier cells builds up this current density rapidly decays to approximately 0.3 ampere per square inch and at the end of the ten minutes the rectifier cells are completely electroformed. During this electroforming period the temperature of the cold oil bath is maintained below 40 C. to limit the temperature rise of the rectifier cells during this period and to accelerate the electroforming operation. After the rectifier cells have been completely electroformed, they are removed from the cold oil bath, and are drained and washed with alcohol to remove the excess oil and are then dried. In this connection it will be understood that suitable circuit arrangements may be provided so that large numbers of cells may be electroformed simultaneously, the circuit diagram shown in Fig. 3 being simplified for purposes of illustration.

The single rectifier cells may also be electroformed by employing a pulsating direct current instead of the alternating current arrangement shown in Fig. 3. Thus, as shown in Fig. 4, the alternating current source is connected to a full wave bridge rectifier 40 which may be of conventional design and develops a pulsating direct current voltage of uniform magnitude between the conductors 41 and 42. A series of rectifier cells '43, 44 and 45 are connected in parallel between the conductors 41 and 42.' The rectifier cells 43, 44 and 45 are connected between the conductors 41 and 42 in-the correct .polarity so that the pulsating direct current voltage is applied in the reverse direction and the voltage E between the conductors 41 and 42 is adjusted to a fixed maximum to provide the above-described initial current density at the start of the electroforming period. Resistor 46, of the order of /2 ohm, is connected in series with the bank of rectifiers to absorb the initial surge of current which occurs when a voltage of "fixed maximum magnitude is applied to the rectifier circuit. In this connection it will be understood that the rectifier cells 43, 44 and 45 are positioned within the above described cold oil bath while pulsating direct current is being applied thereto during the electroforrning period and except for the different circuit arrangement of Fig. 4 the cells are treated identically to that described above in connection with Figs. 1 and 3 prior to and subsequent to the electroforming operation.

The rectifier cells which have been assembled into half wave rectifier stacks may also be electroformed in a manner substantially identical to that described above in detail in connection with the single rectifier cells. Thus, referring to Fig. 5, the half wave rectifier stack 50 which consists of a series of unpainted, stacked rectifier cells 51 to 56, inclusive, is connected in series with a condenser 57 and a resistor 58, of the order of /2 ohm, across an alternating current source and is positioned within the above-described cold oil bath. preferably the condenser 57 has a relatively large capacity in the order of rnfd. The voltage E across the series combination of the rectifier stack 50 and the condenser 57 is adjusted to a constant value to provide the above-described initial current density at the start of the electroforming period which again lasts approximately 10 minutes. The resistor 58 absorbs the initial surge of current which results when a constant and relatively high voltage is applied to the circuit. In this connection it will be understood that the rectifier stack 50 is treated preliminarily in a manner identical to that described above in connection with individual rectifier cells and after the electroforming period the stack is removed from the bath, washed with alcohol and dried.

In .a typical run, a group of selenium rectifier cells having a total area of 72 square inches is arranged in a circuit such as Fig. 4, wherein the resistor 46 is a /2 ohm resistor. After the cells have been soaked in an alkaline oil bath at 70 C. and then are immersed in a neutral oil bath at a temperature below 25 C., a 60 volt D. C. is applied to the circuit. The potential drop across the group of rectifiers initially is about 20 v. and increases very rapidly to about 56' v. For the first few seconds (l15 seconds) there is an extreme temperature peak within the rectifiers, reaching as high as 90 to 120 C., even though the rectifiers are in chilled oil. The temperature peak is soon passed and the rectifiers fall to a temperature within 5 to C., of the temperature of the oil bath. An initial current of about 100 amps. flows through the circuit, causing points of low resistance in the rectifiers to receive a very high current and rapidly burn themselves out. The heat of these low resistance points is localized by the chilled oil so that surrounding areas of the rectifiers are not aifected. The burning out of low resistance points is a desirable feature, in that defects in the rectifiers are eliminated; this desirable result is not attained when electroforming is conducted by conventional methods. The rectifiers are almost completely electroformed within the first minute or two in the foregoing process. More than 90% are completely electroformed in 10 minutes and essentially 100% are completely electroformed in minutes. This method, employed a fixed miximum voltage, a high initial current density, low temperature and uniform time, allows for continuous electroforming on a a mass production scale with little or no supervision and no adjustment of the apparatus after electroformin g is started. The savings in man hours are substantial and the cost of electroforming is significantly decreased.

By electroforming the single rectifier cells or the half wave rectifier stacks in the manner described above the time required for the entire electroforming operation is reduced from a period of from two to four hours to a total time of approximately 15 minutes. In addition to this very apreciable saving in manufacturing time it has been found that the rectifiers which are electroformed in the manner described above have superior life characteristics and operate at cooler plate temperatures than rectifiers which have been electroformed b conventional techniques.

While particular embodiments of the invention have been shown, it will be understood, of course, that the invention is not limited thereto since many modifications may be made and it is therefore contemplated by the appended claims to cover any such modifications as fall within the true spirit and scope of the invention.

What is claimed as new and is desired to be secured by Letters Patent of the United States is:

1. The method of electroforming a dry surface contact rectifier which comprises energizing the rectifier in series with a low resistance at a voltage of constant magnitude and at an initial current density of the order of one to three amperes per square inch while said rectifiers is submerged in an oil bath maintained at a temperature below 40 C. and maintained at appproximately neutrality by an organic amine.

2. The method of electroforming a selenium rectifier which comprises energizing the rectifier in series with a low resistance at a voltage of constant magnitude and at an initial current density of the order of one to three amperes per square inch while said rectifier is submerged in an oil bath maintained at a temperature below 40 C. and maintained at a pH of approximately 7 by an organic amine.

3. The method of electroforming a selenium rectifier which comprises energizing the rectifier in series with a low resistance at a voltage of constant magnitude and at an initial current density of the order of one to three amperes per square inch while said rectifier is sumberged in an oil bath maintained at a temperature below 40 C. and maintained at a pH of approximately 7 by an organic amine.

4. The method of electroforming a selenium rectifier which comprises first soaking the rectifier and a warm slightly alkaline oil bath and thereafter energizing said rectifier in series with a low resistance at a voltage of constant magnitude and at an initial current density of the order of one to three amperes per square inch while said rectifier is submerged in an oil bath maintained at a temperature below 40 C. and at a pH of approximately 7 by an organic amine.

5. The method of electroforming a selenium rectifier which comprises first soaking the rectifier in a warm oil bath maintained at a slight alkalinity by an organic amine and thereafter energizing said rectifier in series with a low resistance at a voltage of constant magnitude and at an initial current density of the order of one to three amperes per square inch while said rectifier is submerged in an oil bath maintained at a temperature below 40 C. and at a pH of approximately 7 by an organic amine.

6. The method of electroforming a selenium rectifier which comprises first soaking the rectifier in an oil bath maintained at a pH from about 7.2 to about 8 with an organic amine and at a temperature of the order of C. and thereafter energizing said rectifier in series with a low resistance to at voltage of constant magnitude and at an initial current density of the order of three amperes per square inch while said rectifier is submarged in an oil bath maintained at a temperature below 40 C. and at a pH of approximately 7 by an organic aliphatic amine.

7. The method of electroforming a selenium rectifier which comprises first soaking the rectifier in an oil bath maintained at a pH from about 7.2 to about 8 with diethylenetriamine and at a temperature of the order of 70 C and thereafter energizing said rectifier in series with a low resistance at a voltage of constant magnitude and at an initial current density of the order of three amperes per square inch while said rectifier is submerged in an oil bath maintained at a temperature below 40 C. and a pH of approximately 7 by diethylenetriamine.

References Cited in the file of this patent UNITED STATES PATENTS 2,471,898 Rau May 31, 1949 2,484,204- Blackburn Oct. 11, 1949 2,521,687 Cameron et al Sept. 12, 1950 

5. THE METHOD OF ELECTROFORMING A SELENIUM RECTIFIER WHICH COMPRISES FIRST SOAKING THE RECTIFIER IN A WARM OIL BATH MAINTAINED AT A SLIGHT ALKALINITY BY AN ORGANIC AMINE AND THEREAFTER ENERGIZING SAID RECTIFIER IN SERIES WITH A LOW RESISTANCE AT A VOLTAGE OF CONSTANT MAGNITUDE AND AT AN INITIAL CURRENT DENSITY OF THE ORDER OF ONE TO 